BANGKOK - A stench from Thailand'sinvolvement in the Vietnam War is coming back to haunt thecountry.

Last February, workers upgrading a runway at an airport in HuaHin, 100 kilometers south of Bangkok, fell ill after uncovering fivedrums of a chemical that many suspect is the deadly Agent Orangeherbicide used by the U.S. Air Force against Vietnamese communists.

While debate continues on whether or not the chemical is reallyAgent Orange, the discovery has thrown up serious questions about the culpability of previousThai regimes in the Vietnam War and the negative legacy the warleft behind.

Adding to public outrage over the issue is the revelation thatthe U.S. Air Force tested the carcinogenic Agent Orange on Thaiterritory prior to its use in Vietnam.

From 1961 to 1971, U.S. warplanes sprayed vast areas in southernVietnam with more than 72 million liters of chemical herbicide, ofwhich a major portion was Agent Orange. More than two millionhectares of inland mangrove forest and agricultural land wasaffected by the spraying, which was undertaken ostensibly todefoliate forests used by anti-U.S. Vietnamese guerillas for cover.

The U.S. embassy in Bangkok, while denying that the chemical foundat Hua Hin had anything to do with Agent Orange, did admitthat the carcinogenic chemical was indeed tested near a Thai Armybase in Prachuab Khiri Khan's Pranburi district, not far from HuaHin.

''The U.S. military had tested Agent Orange in Thailand betweenApril 1964 and June 1965 with the full knowledge of the Thaiauthorities,'' a US embassy spokesman said in Bangkok, two monthsafter the discovery was made, citing declassified CIA reports.

The Thai government, while setting up a committee to identify thechemical, has also stated that the United States should shareresponsibility for the toxic substances and provide part of thecosts that will be incurred in cleaning it up. The Thai ministryof science has estimated that treating the chemical and thecontaminated soil will cost between 15 and 20 million baht.

''Once the identification [of the chemical] has been completed,we will talk to the U.S. about the issue of responsibility,'' PrimeMinister Chuan Leekpai was quoted as saying after a cabinetmeeting to consider the issue.

The Thai government's handling of the issue, however, has comeunder attack from critics who fear that the authorities are tryingto hide the true nature of the chemical found in order to preventaffected people from raising claims for compensation.

''The lack of information from both the Thai and U.S. authoritiesonly goes to heighten public suspicion that they have something tohide,'' said an editorial in the Bangkok based daily The Nation.

It said that by not giving detailed information on the mysterychemical, both the Thai and U.S. authorities have inadvertently putthe health of residents and the environment at risk.

Apart from the immediate area where the chemical was found thereare fears that hundreds of villagers living near the forests whereAgent Orange was tested three decades ago might have beenaffected.

Studies conducted by Vietnamese, Russian and even independent U.S.scientists have shown exposure to Agent Orange as beingresponsible for at least eight disease categories, including softtissue cancer.

According to Vietnamese authorities, as many as two millionpeople were exposed to the toxic chemicals and up to 50,000deformed children have been born to parents exposed to thechemicals.

The long-term impact of Agent Orange has been confirmed recentlyby a four-year series of medical investigations by Canadianresearchers in southern Vietnam, which showed extraordinary levelsof poisonous dioxins in the soil and the food chain and even inchildren's bloodstreams.

''Those responsible for the use of Agent Orange in theVietnam War should be held responsible for cleaning up thepoison,'' says James Forsyth, a British writer and researcher whohas campaigned for international attention to the continuedplight of victims of Agent Orange and of cluster bombs used by the U.S.forces during the war.

In Thailand, apart from the immediate issue of identifying andlimiting the damage caused by the testing and storage here of Agent Orange, there are also prickly issues from the country's dictatorial past that have cropped up.

One question being asked by the media, environmentalists andcitizen groups for example, is about who in previous Thaigovernments was responsible for allowing U.S. testing of AgentOrange on Thai territory.

''The matter cannot be allowed to pass without anyone takingresponsibility for the chemicals found buried at Hua Hin,'' said acommentary in the Thai daily Baan Muang.

Pinning down responsibility for the unwarranted presence andactions of U.S. troops in Thailand may, however, prove to be asensitive task given the fact that almost the entire Thaimilitary and bureaucratic establishment was involved in allowingthe country to be used as a major staging point for over a decadeduring the Vietnam War.

Thailand, then under the dictatorship of Air Field Marshal ThanomKittikachorn, allowed the U.S. to use six air bases to stagemilitary operations against Vietnam.

''Thailand is a direct accessory to the crimes that are committedby the American air force in North and South Vietnam, as well asin Laos,'' said a report by a ''war crimes tribunal'' set up in 1969under the chairmanship of anti-war campaigner and renowned Britishphilosopher Bertrand Russell.

The report, which describing Thailand as ''anAmerican fortress in Asia,'' says that by granting air and naval bases foruse by the United States, Thailand made possible the Americanescalation of the war during the mid-sixties.

Not surprisingly both the Thai Department of Aviationand the Thai Air Force - responsible for all operationsat the Hua Hin airport where the chemicals were found - haveclaimed they have no records of any U..S activity in the area duringthe sixties.

''Using the airport as a center for developing dangerouschemical compounds would not be logistically feasible since HuaHin is so far from Vietnam and not in a remote area,'' claimedChaisak Angkasuwan, deputy director-general of the ThaiDepartment of Aviation.

Given such fuzziness regarding the facts, perhaps loss ofhistorical memory may have to be added to the list of diseases thatthe dreaded Agent Orange is supposed to cause in human beings.